Fluid storage systems have been in existence for many years, specifically underground storage systems for the collection and storage of water. While water is collected underground for various reasons, over the past 20 years there has been increased focus on collecting and storing storm water runoff. This is done because of two main concerns. The quantity of storm water runoff is a concern because larger volumes of associated runoff can cause erosion and flooding. Quality of stormwater runoff is a concern because storm water runoff flows into our rivers, streams, lakes, wetlands, and/or oceans. Larger volumes of polluted storm water runoff flowing into such bodies of water can have significant adverse effects on the health of ecosystems.
The Clean Water Act of 1972 enacted laws to improve water infrastructure and quality. Storm water runoff is the major contributor to non-point source pollution. Studies have revealed that contaminated storm water runoff is the leading cause of pollution to our waterways. As we build houses, buildings, parking lots, roads, and other impervious surfaces, we increase the amount of water that runs into our storm water drainage systems and eventually flows into rivers, lakes, streams, wetlands, and/or oceans. As more land becomes impervious, less rain seeps into the ground, resulting in less groundwater recharge and higher velocity surface flows, which cause erosion and increased pollution levels of water bodies and the environment.
To combat these storm water challenges associated with urbanization storm water detention and retention methods have been developed to help mitigate the impact of increased runoff. Historically, open detention basins, wetlands, ponds or other open systems have been employed to capture storm water runoff with the intention of detaining and slowly releasing downstream over time at low flows using outlet flow controls, storing and slowly infiltrating back into the soils below to maximize groundwater recharge or retain and use for irrigation or other recycled water needs. While the open systems are very effective and efficient the cost of the land associated with these system can make them prohibitive. In areas such as cities or more densely populated suburbs the cost of land or availability of space has become limited. In these areas many developers and municipalities have turned to the use of underground storage systems which allow roads, parking lots, and building to be placed over the top of them.
A wide range of underground storage systems exist, specifically for the storage of storm water runoff. Arrays of pipes, placed side-by-side are used to store water. Pipe systems made of concrete, plastic or corrugated steel have been used. More recently arched plastic chamber systems have been in use. As with pipes, rock backfill is used to fill the space surrounding them to create added void areas for storing additional water along with providing additional structural reinforcement.
In general, these types of systems require at least one foot of rock backfill over the top and at least one or more feet of additional native soil over the top to support the loading associated with vehicles on streets and parking lots. These systems also require rock backfill of a foot or more around their perimeter sides to provide structural reinforcement due to lateral loading associated with soil pressure.
Lastly, these systems most also be placed on a rock base for structural support. Because these system are round or arched a substantial amount of rock backfill must be used to surround them and placed in between them. As such, the amount of void space available for storing water compared to the amount of soil required to be excavated is only around 60%.
Over time, plastic and concrete rectangular or cube shaped modular systems were developed that more efficiently stored storm water because the modules could be placed side-to-side and end-to-end without the need for additional rock backfill to be placed between each module as found with pipe and arched systems. With these rectangular and cube shaped systems the void space available for storing water compared to the amount of soil required to be excavated is up to 90% plus. While plastic type rectangular and cubes systems still require at two feet of rock backfill over the top, two feet round the perimeter sides, and six inches underneath to handle downward and lateral loading, the concrete rectangular and cubed systems do not.
Concrete rectangular or cubed modular systems have the benefit of not requiring rock backfill over the top or surrounding the sides because of their additional strength when compared to plastic systems. Yet, these rectangular or cubed concrete structures still have depth limitations due to the lateral loading associated with soil pressure.
For example, currently available concrete systems cannot have the bottom of the structure be deeper than eighteen feet below surface level without modifying the standard wall thickness of the structure from six inches to eight inches or more plus adding additional rebar reinforcement. Doing so adds cost, weight and complexity to design. This inherent design limitation is related directly to the shape and design of these structures.
Concrete rectangular or cube shaped structures have five sides, four vertically extending walls and a bottom or top side. One side must be open because of how pre-cast concrete molds are made and how the concrete structure is pulled from the mold. At least one side of the concrete structure must be missing for it to be pulled from the metal mold that consists of inner and outer walls and either a top or bottom side.
Unfortunately this missing side, required for manufacturing, creates an inherent weak point for the walls. The middle of each wall, especially the longer walls for rectangular structures, where the wall meets the end of the missing top or bottom side has no perpendicular connection as with the opposite side of the same wall where it connects to the top or bottom side. This weak point on the center of each wall at the open end is the reason why these systems have depth limitations. This is known as deflection. This weak point becomes further exaggerated the taller the wall becomes and the longer it becomes; the further away it is from the perpendicular connecting floor or adjacent wall on the opposite end. Therefore, taller systems which extend down deeper from the surface underground run into a compounding problem of taller walls and increased lateral loading (soil pressure).
Furthermore, there are also equipment limitations with concrete rectangular or cubed shaped structures. Most precast concrete plants utilize an overhead crane inside a metal building. The height of this crane is a limitation on how tall a single five sided, four walls and a top or bottom side, structure can be. The process of pulling a concrete casting from the mold requires it to be pulled up from the mold, opposite of the open side, sliding the walls out from between the inner and out mold walls.
Because of this method, generally the walls of these concrete structures are not greater than seven feet tall. Therefore, in order to make a taller overall structure, two shorter structures must be stacked on top of each other in a “clamshell” configuration with open ends facing each other so that the joined structure has one top and one bottom. Once again, the weak point being in the middle of each wall, horizontally, on the end opposite of the perpendicular connecting top or bottom side.
Lastly, current designs of concrete rectangular or cubed shaped structures, have limitations related to shipping, primarily on large flatbed trucks. These trucks have transportation limits on weight, length, width and height. Standard flatbed trucks are forty feet long. A standard load width is eight feet. Wide load up to twelve feet. Anything wider requires pilot cars and an escort which is very expensive. Also, height limitations are generally eight feet to be transported on most interstates due to overpasses. Standard weight limitations are forty-five thousand pounds. When designing these structures it is important to make the structure as large as possible without exceeding the shipping limitations to maximize feasibility due to economies of scale.
As explained, current design of underground systems have limitations related to loads from above and from the sides. These systems must be designed without risk of cracking, collapsing or other types of structural failure. Concrete rectangular or cubed structures have inherent weak points which limit the depth at which they are installed with standard wall thicknesses and design. The inherent flaw is related to the basic shape of the structure which has walls running perpendicular and parallel to one another.
The need for a system that overcomes these inherent shape related limitations is evident. The solution lies within utilizing principles of biomimetics and studying efficient structures found in nature and utilizing these more efficient natural shapes in combination with current precast concrete design processes to create a system that overcomes the limitations of the current available technologies.
One of the most efficient structures in nature is the honeycomb which is found in beehives, honeycomb weathering in rocks, tripe and bone. The related hexagon shape has been found to make the most efficient use of space and building materials. Throughout history this structure has been admired to be very light, strong and structurally efficient. While this technology has been applied to paper products, composite materials, metals like aluminum, plastics, and carbon nanotubes it has never been applied to modular precast concrete structures, let alone structures used for the underground storage of storm water or other fluids.